An antenna is a transducer, which transmits or receives electromagnetic waves. Antennas include one or more elements, which are conductors that radiate the electromagnetic waves (radiators). When transmitting, an alternating current is created in the element(s) by application of a voltage at the terminals of the antenna, which causes the element(s) to radiate an electromagnetic field. When receiving, an electromagnetic field from a remote source induces an alternating current in the elements generating a corresponding voltage at the terminals of the antenna.
The orientation of the electric field of the radio wave with respect to the Earth's surface is called the polarization of an antenna. Polarization of an antenna is typically determined by the physical structure and orientation of the antenna. For example, a straight wire antenna may have one polarization when mounted vertically, and a different polarization when mounted horizontally. In other words, polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the direction of motion of the radio wave. In some cases, polarization may be elliptical (the projection is oblong), meaning that the antenna varies over time in the polarization of the radio waves it is emitting. In other cases, polarization may be linear (the ellipse collapses into a line), or circular (in which the ellipse varies maximally). In linear polarization the antenna compels the electric field of the emitted radio wave to a particular orientation, such as horizontal and vertical polarization. Alternatively, polarization may be circular, in which the antenna continuously varies the electric field of the radio wave through all possible values of its orientation with regard to the Earth's surface.
In practice, it is important that linearly polarized antennas be matched to substantially reduce the received signal strength requirement. Accordingly, a horizontal polarization works best with a substantially horizontal polarization antenna and vertical polarization antenna works best with a substantially vertical polarization antenna. Intermediate matchings will lose some signal strength, but not as much as a complete mismatch.
Furthermore, because the electro-magnetic wave travels through different parts of the antenna system (radio, feed line, antenna, free space, etc.), it may encounter differences in impedance. At each interface, depending on how well the impedance is matched, some portion of the wave's energy reflects back to the source of the wave, forming a standing wave in the feed line. Impedance matching deals with minimizing impedance differences at each interface to reduce ratio of maximum power to minimum power, that is, the standing wave ratio (SWR), and to maximize power transfer through each part of the antenna system.
Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, for example, by adjusting the length and width of the feed line.
Many antenna applications require broadband, dual polarized antenna elements to transmit and/or receive a diverse number of polarizations and hence the receiver antenna must be able to handle multiple polarizations. Moreover, sometimes the sensor location does not easily lend itself to providing a particular polarization, like an element that is located 60 degrees off center on a cylinder yet needs to be able to transmit and/or receive a horizontally polarized signal. Furthermore, many antenna applications do not have much depth requiring conformal mounting and collocation of the orthogonally polarized antennas.
Prior attempts to solve the above mentioned problems include a quad-notch in a cavity. The quad-notch in a cavity offers two orthogonal polarizations that is broadband (˜9:1) and high gain. However, the cavity and antenna require a large amount of space (approximately 12×12×3 inches deep for a 2-18 GHz antenna), which is too large for some applications. A conventional conformal channel monopole provides a thin (approximately 2×1×0.025 for a 2-18 GHz antenna), conformal antenna that is also broadband (˜9:1). However, it only provides one polarization at any given location. On the other hand, antennas with ultra-wide bandwidth have usually been too large to consider for many applications, such as antenna arrays.